Actually I believe this is backwards. Adobe RGB should be _MG and sRGB should be IMG.

Also keep in mind that if you shoot in Auto (Basic) mode the file will be IMG regardless of the color space that you have set for the camera. If you have Adobe RGB set and shoot in Program mode the file will be _IMG. Change the dial to Auto and the file will be IMG. The attached snip show two files shot right after each other. The first is in Program and the second is in Auto mode. Both were shot in RAW format.